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AT85C51SND3Bx

7632A–MP3–03/06

Clock Controller The AT85C51SND3Bx clock controller is based on an on-chip oscillator feeding two on-

chip Phase Lock Loop (PLL) dedicated for the USB controller (see

Section “USB Con-

troller”, page 85) and the Audio Controller (see Section “Audio Controller”, page 149).

All internal clocks to the peripherals and CPU core are generated by this controller.

Oscillator X1 and X2 pins of AT85C51SND3Bx are the input and the output of a frequency power-

optimized single-stage on-chip inverter (see

Figure 15) that can be configured with off-

chip components such as a Pierce oscillator (see Figure 16). Value of capacitors and

crystal characteristics are detailed in the Section “DC Characteristics”, page 241.

Authorized frequency In order to be able to be able to properly detect the oscillating frequency when in In Sys-

tem Programming mode and then generate the 480MHz requested for USB connection,

only the following frequencies are authorized:

12MHz, 13MHz, 16MHz, 19.2MHz, 19.5MHz, 20MHz, 24MHz and 26MHz.

Power Optimization In order to optimize the power consumption, oscillator gain can be adjusted by software

depending on the crystal frequency. Such optimization is done after reset using

OSCF1:0 bits in CKCON register (see

Table 31) according to Table 23. Moreover if

external frequency signal is input (X1 driven by a remote host) it is possible to switch off

the internal amplifier by setting the OSCAMP bit in CKCON register as shown in

Figure 15.

Table 23. Oscillator Frequency Configuration

The oscillator outputs a clock: the oscillator clock used to feed the clock generator and

the system clock generator.

The oscillator clock can be disabled by entering the power-down reduction mode as

detailed in the

Section “Power Management”, page 18.

Figure 15. Oscillator Block Diagram and Symbol

OSCF1:0 Crystal Clock Frequency Range (F

OSC

)

00 22 - 26 MHz (default)

01 18 - 22 MHz

10 14 - 18 MHz.

11 10 - 14 MHz

OSCF1:0

CKCON4:3

PCON.1

OSC

CLOCK

Oscillator Clock Symbol

Oscillator

Clock

OSCAMP

CKCON.5

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 262 263

Atmel AT85DVK-07 Especificaciones Pagina 27

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